Since all electronic devices require electric energy to maintain the working state, an electric power system is provided to convert the received electric energy into regulated voltages for these electronic devices. As known, it is important to stabilize the electric power system. For example, since the electric power system in the ship is somewhat different from the electric power system on the shore, the demands on the reliability and stability of the electric power system in the ship industry are more stringent. Particularly, when the electric motor of the ship is in an acceleration stage or the sea condition is worse, the loading of the electric motor is increased and thus the required power of the electric motor is increased. If the performance of the electric power system is poor, the electric power system cannot make an immediate response. Under this circumstance, the propulsive power of the electric motor is insufficient. Moreover, since the DC bus voltage of a variable-frequency drive in the electric power system is decreased and the operation of the electric motor is unstable, the propulsive performance is deteriorated. Whereas, when the electric motor is in a deceleration stage and the electric motor is in a power generation mode, the propulsive power is quickly decreased. Under this circumstance, a portion of the electric energy is fed back from the electric motor to the DC bus of the variable-frequency drive of the electric power system. Since the bus voltage of the DC bus is abruptly increased, the electronic components of the electric power system are possibly damaged.
Conventionally, some approaches are provided to solving the problems about the abruptly increased demand of the propulsive power. For example, the output power of the electric power system is increased or a backup power generator is additionally equipped to assure the utilization flexibility. However, as the output power of the electric power system is increased, the thermal power generated by the electric motor also increases. Under this circumstance, the operating efficiency of the electric power system is decreased and the fabricating cost is increased. Moreover, because of the working characteristics of the power generator, the power generator cannot make an immediate response to the abruptly increased demand of the propulsive power. In other words, the installation of the backup power generator cannot comply with the requirement of utilization flexibility.
Similarly, some approaches are provided to solving the problems about the abruptly decreased demand of the propulsive power. For example, the electric power system is equipped with an additional bleeder resistor to consume the excess electric energy, or the excess electric energy is recycled to a power grid. These approaches have restrictions. In case that the bleeder resistor is used, the size, weight and heat-dissipating efficiency of the bleeder resistor have to be taken into consideration and the operating efficiency of the electric power system is adversely affected by the energy consumption. Moreover, since the capacity of the power grid integrated the electric power system is usually very low, the excess electric energy recycled to the power grid may influence the stability of the power grid.
Moreover, for reducing the pollution to ports in some areas, ships that pull in to a shore or harbor are permitted to use light oil as the source of the power generator or use the shore power to guide the ship to dock. However, the use of the light oil or the shore power increases the operating cost of the ship.
Therefore, there is a need of providing an electric power system and a control method in order to overcome the above drawbacks.